SAE Technical Paper 930141 published Mar. 1, 1993, entitled A Sampling System for the Measurement of PreCatalyst Emissions from Vehicles Operating Under Transient Conditions provides a full description of such a system. That system incorporates a so called "mini-diluter" utilizing a pair of mass flow controllers externally controlled to provide a mixture of exhaust gas and diluent gas in a prescribed ratio and at a flow rate related to the flow of exhaust through the vehicle. The mixture of diluted exhaust and diluent is provided to a plurality of bag samplers for collection of samples.
FIG. 1 illustrates the mini-diluter portion of this prior art sampling system as described in the SAE paper. It shows the appropriately labeled exhaust gas pipe (exhaust), an exhaust mass flow controller (MFM) actuated by a control circuit and exhaust flow signal, a diluent mass flow controller (MFM) connected with a source of zero air, a sample pump (metal bellows pump) and three sample bags connected with the pump.
While the mini-diluter sampling system discussed has been successfully utilized, it is found necessary to periodically check the calibrations of the two mass flow controllers at a number of points across their complete range of operation and to establish, at least initially, the relative accuracy of the flow rates of the two mass flow controllers over the range of flow conditions. Because the flow rates of the two mass flow controllers at various flow settings are not linear and may have differing response times, the repeated calibration of these devices requires considerable time and the resulting accuracy of the measurements is limited, although adequate for the purposes intended.